The next generation materials and components used in molecular circuitry, optical applications, clinical diagnostics, sensors, and drug delivery devices will rely on building blocks self-assembling and forming higher order structures. But despite recent advancements in the synthesis techniques of a large variety of organic and inorganic materials, assembling them into ordered structures having functional capabilities remains the major bottle-neck. Within this context, inhomogeneous particles have attracted tremendous amount of attention, recently. They can be defined as particles with well defined sites, patches or inhomogeneities consisting of a separate chemical entity at prescribed locations. Such particles have been demonstrated to serve as much more efficient building blocks to create premeditated, higher-order complex architectures owing to their built-in recognition sites to direct the self-organization of particles (Zhang et al., 2005, Glotzer, 2004, Zhang and Glotzer, 2004). Structures thus formed can take the shapes of sheets, diamonds, rings, pyramids, and chains, and have potential applications in self-assembling electronic circuits, photonic crystals, solar panels, biological applications and clinical diagnosis utilizing selective molecular recognition, scaffolds for assembling other compounds, as colloidal liquid crystals in display devices, electro-rheological fluids, and switching devices (Cayre et al., 2003, Lu et al., 2003, Nakahama et al., 2000, Takei and Shimizu, 1997).
Colloidal particles (100 nm to 10 micron in diameter) usually have their surface uniformly covered with charged species or any other molecular species which is either ionically bound or covalently attached. Synthesis of particles with geometrically well-defined and precisely located inhomogeneities is a challenging task for surface and colloid scientists. There are only a few techniques to fabricate inhomogeneous particles: Langmuir-Blodgett technique, microcontact printing, evaporation (physical vapor deposition) of metals on colloid monolayer followed by chemisorption, using either gas-liquid, liquid-solid, or gas-solid interface to create particles with hemispheres of two different functionalities, and simultaneous electrohydrodynamic jetting.
In the Langmuir-Blodgett technique, a plate covered with a monolayer of colloids is initially dipped in a solvent. The solvent also hosts a floating monolayer of the coating polymer. The plate is slowly pulled upwards perpendicular to the monolayer of the coating polymer. The monolayer of coating polymer gets transferred onto a restrictive part of the monolayer of colloidal particles. Thus, the individual colloidal particles end up having inhomogeneities of the polymer. Nakahama et al. demonstrated this by coating a monolayer of amphiphilic terpolymer of octadecyl acrylamide, p-nitrophenyl acrylate, and 2,2,2-trifluoroethyl methacrylate on particles approximately 182 nm in diameter initially deposited on a glass plate (Nakahama et al., 2000).
In the microcontact printing technique, films of specific chemicals deposited on PDMS stamps are printed onto monolayer of colloid particles deposited on a solid substrate. In one particular example, Cayre et al. prepared a monolayer of latex spheres on a glass substrate. Then, a monolayer film of water-insoluble surfactant with charge opposite to that of latex particles was deposited on a PDMS stamp. Lastly, the surfactant film is printed onto the colloid monolayer and then the colloids are redispersed in water (Cayre et al., 2003). In this way, the colloidal particles end up having a bipolar surface charge distribution.
In the vapor deposition technique, the monodisperse colloidal particles are either spin coated (Choi et al., 2003) or drop-casted (Love et al., 2002) on a solid substrate to form a monolayer. Subsequent physical vapor deposition of metals (e.g., gold, platinum, and palladium) coats only the top hemisphere of the particles (Petit et al., 2001, Takei and Shimizu, 1997).
Interface between two media has also been used to introduce functional dissymmetry in uniform colloidal particles (Petit et al., 2000, Fujimoto et al., 1999). In a typically process, the colloidal particles are first assembled on the media interface (air-liquid or liquid-solid) to form a monolayer. The two hemispheres are exposed to the two different mediums owing to which the two sides will react differently. Functional groups to be impinged on either of the hemispheres of the colloid, is introduced through one of the two mediums.
In the technique of simultaneous electrohydrodynamic jetting, two distinct polymer solutions were pumped through a modified nozzle with a side-by-side geometry. The ejecting liquids form a Taylor cone which was fragmented to give particles with two distinct hemispheres (also called as biphasic colloids) (Roh et al., 2005).
Above mentioned particles with dual surface functionalities have also been called “Janus” particles (named after the two-faced Roman god Janus) and anisotropic particles.
All these preparation routes to inhomogeneous particles tend to be labor-intensive processes, requiring multiple steps to be performed in a sequential manner. The present invention combines all the processing benefits of nanoparticle assembly of particles with the unusual phase-separating behavior of polymers that have the same charge but different molecular structures.